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A little nuclear physics ©1996, Institut Laue-Langevin The neutron bottle Our understanding of the genesis of the universe ("Big Bang"theory), of physics in general and even more down to earth problems, is based on our knowledge of universal forces (gravitation, electromagnetic, strong and weak electric forces) and several fundamental physical constants. Some of these constants are known with very high precision (speed of light ...), whereas others are known only very roughly and handicap the theoretical understanding of certain phenomena observed. Considerable effort in laboratories throughout the world is invested to improve this knowledge. The ILL is present in this competition with experiments such as the neutron bottle, which has enabled the weak interaction between quarks to be studied by measuring the lifetime of the neutron and to define more precisely the electric dipole moment of the neutron. Shortly after the "Big Bang", neutrons, protons, and electrons, the constituents of atoms (which themselves constitute matter), were formed by assembling even smaller particles known as quarks, under the action of the so-called "weak electric force". Electrons are generally considered to be eternal, as are protons (well, almost, with a lifetime greater than 1032 years). Neutrons are however unstable, but they have survived from the "Big Bang" until now in the nuclei of atoms due to the forces (bonds) which hold them. A free neutron however breaks up spontaneously after an average lifetime of only 15 minutes (Fig. 1). The composition of the neutron in quarks is "udd" ("u" for up and "d" for down) whereas it is "udu" for the proton. The spontaneous disintegration of the neutron therefore corresponds to the transformation of a "d" quark into a "u" quark under the effect of the weak force between quarks. Figure 1. Spontaneous disintegration of a free neutron. Measuring the lifetime of the free neutron is therefore a good way of measuring the weak force which is still not well known. Measuring the lifetime of the neutron The lifetime of the neutron is quite difficult to measure with precision. in fact, today's neutron "lamps" (nuclear reactors, spallation sources) produce neutrons travelling at such speed that a detector one metre long through which 2 million neutrons pass would observe only one disintegration. To improve on this, it would be preferable, talking schematically, to slow down or even stop a certain number of neutrons, put them in a bottle, and calmly observe their disappearance over half an hour. How can we put neutrons in a bottle? Neutrons produced by a reactor are travelling very fast. They pass through materials with great ease, and it is not possible to contain them with material walls. On the other hand a magnetic bottle has been used successfully (storage ring for neutrons). Other techniques have been tried throughout the world, but that chosen by the ILL consists in creating a flow of very slow neutrons (cold neutrons with very low energies and very long wavelengths). These very slow neutrons can be made to rebound indefinitely from the walls of perfect mirrors (Figs. 2 & 3). In fact in the same way that a prism deviates red light more than blue light, slower neutrons are deviated (refracted) more than fast ones by the material through which they travel, and the slowest are more easily reflected by a surface. U l t r a - c o l d neutrons (the slowest) are even totally reflected by certain surfaces, that is, once they are put into a closed volume, a bottle, they can never get out. extrapolated to the case for an infinitely large sized bottle, completely free of perturbations introduced by the container. Ultra-cold neutrons Figure 2. Diagram of the neutron bottle at the ILL. This kind of bottle, made of a solid material ( glass or metal), would unfortunately have "leaks" due to cracks, invisible to us, but extremely large for a particle only 10-13 cm in diameter, but also because of a radiator effect due mainly to hydrogen atoms, always present, which warm up or absorb neutrons with which they collide. The trick was to cover the walls with a special hydrogen-free oil (no cracks and no absorption). The last trick to obtain very precise results, was to make a bottle whose volume could be changed. For a given number of neutrons introduced into the bottle, the bigger the bottle the smaller the number of collisions between the neutrons and the walls. This enabled the experimental results to be How can we produce a sufficient quantity of ultracold neutrons? The ILL reactor is the worlds' most intense source of "thermal" neutrons. By letting them pass through a sphere of 20 litres of liquid deuterium (an isotope of hydrogen) at - 253 °C we obtain cold (slow) neutrons. The slowest of these are selected by using a curved guide (the neutron analogue of optical fibres for light). These neutrons have an average speed of 50 m/s, still too fast to be kept in the bottle. They are directed towards a turbine (similar to hydro-electric turbine), where by successive rebounds on the blades turning at 25 m/s, their velocity is almost cancelled (Fig. 3). They can now be put in the bottle. The final chapter A long series of measurements made with the bottle in figure 2 has given a lifetime for the neutron of 887.6 ± 3 s, which combined with other results published over the past 6 years gives a world average of 885.9 ± 1.6 seconds. The theoreticiens would like a precision of better than one second. We intend to improve the neutron bottle by taking into account the effect of gravity on the ultra-cold neutrons. ❃❅❃ Figure 3. Diagram of the complete instrument.